US4041538A - Low noise magnetic transducer preamplifier having flat response - Google Patents

Low noise magnetic transducer preamplifier having flat response Download PDF

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Publication number
US4041538A
US4041538A US05/676,237 US67623776A US4041538A US 4041538 A US4041538 A US 4041538A US 67623776 A US67623776 A US 67623776A US 4041538 A US4041538 A US 4041538A
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United States
Prior art keywords
preamplifier
transducer
feedback loop
capacitor
input terminal
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Expired - Lifetime
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US05/676,237
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English (en)
Inventor
Robert R. Parker
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Bell and Howell Co
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Bell and Howell Co
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Publication date
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Priority to US05/676,237 priority Critical patent/US4041538A/en
Priority to JP4047677A priority patent/JPS52131708A/ja
Application granted granted Critical
Publication of US4041538A publication Critical patent/US4041538A/en
Priority to JP1985099009U priority patent/JPS6121009U/ja
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
    • G11B5/02—Recording, reproducing, or erasing methods; Read, write or erase circuits therefor
    • G11B5/027—Analogue recording

Definitions

  • This invention relates to a recording machine playback amplifier, and, in particular, to a circuit having an improved signal-to-noise ratio.
  • An object of the present invention is to provide a new and improved playback preamplifier having flat output response characteristics and an improved signal-to-noise ratio.
  • the present invention includes a preamplifier circuit for use on a recording machine which provides a substantially flat response between a general range of 200 and 6000 Hz. and has a substantially improved signal-to-noise ratio.
  • the circuit includes a three stage amplifier having an overall DC feedback loop. The magnetic transducer is connected in the feedback loop so that the DC feedback current biases the transducer with the DC feedback current.
  • FIG. 1 is an electrical diagram of the playback preamplifier of the present invention
  • FIG. 2 is a graph of the transducer response curve as a function of frequency
  • FIG. 3 is a similar graph of the preamplifier response as a function of frequency
  • FIG. 4 is a graph representing the overall combined output from the circuit.
  • the playback preamplifier circuit, generally designated 10, of the present invention is shown in FIG. 1.
  • the circuit is designed for use with a recording/playback machine which utilizes a magnetic recording medium. Therefore, the circuit is suitable for reel-to-reel tape recorders, cassette recorders, and 8-track units.
  • the circuit is used in a card reading type teaching machine such as that shown and described in U.S. Pat. No. 3,869,718 assigned to the assignee of the present invention, which patent is hereby incorporated by reference.
  • the playback preamplifier is the subject of the present invention and therefore the details and elements used therewith in the recording mode have been deleted in an attempt to simplify the present disclosure in order to clearly point out the present invention.
  • the circuit includes a three stage amplifier comprising transistors Q1, Q2 and Q3.
  • the transistors Q1 and Q3 are of the NPN type such as transistor 2N5089 while Q2 is a PNP type transistor such as 2N5087.
  • the transistors are cascaded collector to base as shown.
  • the output from the preamplifier is taken from an output terminal 12 through a capacitor 14 to a subsequent power amplifier.
  • a variable resistor may be used to provide a volume control for the sound output.
  • the preamplifier is powered by a 4.5 volt DC source, shown at the top of the drawing, through a resistor 16 to the output terminal 12 of the preamplifier 10.
  • a feedback loop, generally designated 18, connects the output terminal 12 with the base of transistor Q1 which defines the input terminal.
  • the "turn on time" of the preamplifier 10 is determined by the time required to charge capacitor 20 and can be substantially reduced by the use of a Zener diode 22 in the DC feedback loop.
  • a suitable Zener diode is 1N749A which has a Zener voltage of 4.3volts. At turn on, 4.3 volts are impressed on the Zener diode as the capacitor 20 charges to 0.72 volts.
  • the voltage across the Zener diode 22 drops to 1.2 volts, effectively open circuiting the Zener diode and inserting the resistor 24 in the feedback loop circuit 18.
  • the turn on time for the preamplifier 10 as shown is approximately 180 milliseconds while the leakage through the Zener diode 22 is minute, approximately 1 micro amp, compared to the 45 micro amps through the resistor 24 after charging of the capacitor 20.
  • the capacitor 20 is connected in parallel with a 12 k resistor 26 between the feedback loop and ground to maintain 0.72 volts of DC current in the feedback loop during stable operation.
  • a 10 ohm resistor 28 is serially connected with a magnetic transducer 30 and the base of transistor Q1 to complete the feedback loop.
  • approximately 0.2 micro amps of DC current through the head coil provides a very small DC head bias.
  • the preamplifier 10 is used on a teaching machine in which oral signals are recorded and played back to a student user.
  • preamplifier 10 is designed to provide a substantially flat response for a target frequency range generally between 200 and 6000 Hz. and any signals having a substantially higher frequency are considered to be unimportant. It has been found in testing that the 0.2 micro amp DC head bias does not affect or deteriorate any signals on the recording medium within the target frequency response but has been found to provide a beneficial signal-to-noise ratio improvement.
  • FIGS. 2, 3 and 4 represent voltage output of the individual components as a function of frequency.
  • FIG. 2 shows the output of the transducer as a function of frequency which is initially very small and increases to a peak output at approximately 2000 Hz.
  • FIG. 3 shows the output of the three stage preamplifier.
  • the particular response curve chosen has a peak output at approximately 200 Hz. which decreases rapidly to a minimum at 2000 Hz. as shown on the left of the graph.
  • a capacitor 36 is connected between the base of Q1 and ground. The value of the capacitor 36 is 0.0022 micro farads. This capacitor in combination with the transducer 30 provides a large voltage peak at resonance which greatly increases the transducer output beginning at approximately 2,000 Hz. to a peak value at approximately 6,700 Hz. and decreases rapidly thereafter.
  • the combined curve for the preamplifier and head is shown in FIG. 4 to be substantially flat between 200 and 6000.
  • a second feedback loop, generally designated 40, around transistors Q2 and Q3 provides the low frequency boost.
  • the feedback circuit 40 is connected between the output terminal 12 and the emitter of Q2 and includes a 10 kilo ohm resistor 42 and a 0.022 micro farad capacitor 44.
  • the ohm values of the remaining resistors in the preamplifier circuit 10 are shown in the drawings as well as the values of the other components and need not be explained in detail herein.
  • the signal-to-noise ratio of the overall preamplifier circuit 10 has been found to be 65 db. This is a significant improvement, aproximately 8 db over prior art preamplifiers. The increased signal-to-noise ratio is due to a combination of factors, all of which are not completely understood.
  • a conventional preamplifier of this type would normally include a feedback resistor across the collector and base of Q1 and an appropriate input base resistor between the base of Q1 and ground. By providing a feedback loop around all of the transistors Q1, Q2 and Q3, these conventional resistors have been eliminated.
  • Resistors of this type normally produce a significant amount of internal noise which lowers the sound-to-noise ratio, and their removal enhances the signal-to-noise ratio of the entire circuit. Furthermore, the elimination of the input base resistor provides a substantially higher input impedance to Q1, approximately 50 times higher than with a conventional preamplifier. The DC current in the head is believed to oppose some noise generating components and additionally enhance the overall signal-to-noise ratio. Therefore, the preamplifier 10 provides a flat response in the target frequency range while maintaining a very respectable signal-to-noise ratio.

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  • Amplifiers (AREA)
US05/676,237 1976-04-12 1976-04-12 Low noise magnetic transducer preamplifier having flat response Expired - Lifetime US4041538A (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
US05/676,237 US4041538A (en) 1976-04-12 1976-04-12 Low noise magnetic transducer preamplifier having flat response
JP4047677A JPS52131708A (en) 1976-04-12 1977-04-11 Regenerative preamplifier
JP1985099009U JPS6121009U (ja) 1976-04-12 1985-06-28 再生前置増幅器

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US05/676,237 US4041538A (en) 1976-04-12 1976-04-12 Low noise magnetic transducer preamplifier having flat response

Publications (1)

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US4041538A true US4041538A (en) 1977-08-09

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US (1) US4041538A (forum.php)
JP (2) JPS52131708A (forum.php)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4134140A (en) * 1976-10-01 1979-01-09 Eastman Technology, Inc. Voltage mode amplifier for use with a high Q magnetic head
US4210942A (en) * 1977-03-11 1980-07-01 Hitachi, Ltd. Video signal play-back circuit
US4255769A (en) * 1979-06-28 1981-03-10 American Optical Corporation Low-noise preamplifier
FR2479529A1 (fr) * 1980-03-28 1981-10-02 Hitachi Ltd Appareil de lecture de bande magnetique
US4470020A (en) * 1982-05-06 1984-09-04 Mohr Daniel R Virtual ground preamplifier for magnetic phono cartridge
WO1989012886A1 (en) * 1988-06-21 1989-12-28 The Governors Of Paisley College Of Technology Improvements in magnetic recording
US5309295A (en) * 1992-10-08 1994-05-03 International Business Machines Corporation Method and apparatus for biasing a magneto-resistive head

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0644326B2 (ja) * 1984-07-06 1994-06-08 ソニー株式会社 磁気記録再生装置

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3027549A (en) * 1956-06-06 1962-03-27 Ibm Magnetic transducer
US3530257A (en) * 1968-07-22 1970-09-22 Teledyne Geotech Modulation of an oscillatory circuit for driving a magnetic recording head
US3564160A (en) * 1968-10-11 1971-02-16 Ampex Dc bias servomethod and apparatus for magnetic recording heads
US3666893A (en) * 1969-09-25 1972-05-30 Tokyo Shibaura Electric Co Magnetic eraser with head winding as part of oscillator circuit
US3683131A (en) * 1965-06-28 1972-08-08 Bell & Howell Co Magnetic tape recording circuit
US3838452A (en) * 1972-12-07 1974-09-24 Honeywell Inc Recording amplifier for bias-type magnetic recording

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB851052A (en) * 1955-12-31 1960-10-12 Telefunken Gmbh Improvements relating to the coupling of an electro-magnetic transducing head or similar signal source to a transistor amplifier
JPS5330261Y2 (forum.php) * 1972-08-28 1978-07-28
JPS5134401Y2 (forum.php) * 1973-06-13 1976-08-25

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3027549A (en) * 1956-06-06 1962-03-27 Ibm Magnetic transducer
US3683131A (en) * 1965-06-28 1972-08-08 Bell & Howell Co Magnetic tape recording circuit
US3530257A (en) * 1968-07-22 1970-09-22 Teledyne Geotech Modulation of an oscillatory circuit for driving a magnetic recording head
US3564160A (en) * 1968-10-11 1971-02-16 Ampex Dc bias servomethod and apparatus for magnetic recording heads
US3666893A (en) * 1969-09-25 1972-05-30 Tokyo Shibaura Electric Co Magnetic eraser with head winding as part of oscillator circuit
US3838452A (en) * 1972-12-07 1974-09-24 Honeywell Inc Recording amplifier for bias-type magnetic recording

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
Transistor Tape Preamplifier - Wireless World, 12/58, p. 572, "Use of Head Inductance in a Feedback Circuit", Ridler. *

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4134140A (en) * 1976-10-01 1979-01-09 Eastman Technology, Inc. Voltage mode amplifier for use with a high Q magnetic head
US4210942A (en) * 1977-03-11 1980-07-01 Hitachi, Ltd. Video signal play-back circuit
USRE32132E (en) * 1977-03-11 1986-04-29 Hitachi, Ltd. Video signal play-back circuit
US4255769A (en) * 1979-06-28 1981-03-10 American Optical Corporation Low-noise preamplifier
FR2479529A1 (fr) * 1980-03-28 1981-10-02 Hitachi Ltd Appareil de lecture de bande magnetique
US4470020A (en) * 1982-05-06 1984-09-04 Mohr Daniel R Virtual ground preamplifier for magnetic phono cartridge
WO1989012886A1 (en) * 1988-06-21 1989-12-28 The Governors Of Paisley College Of Technology Improvements in magnetic recording
AU640612B2 (en) * 1988-06-21 1993-09-02 Governors Of Paisley College Of Technology, The Improvements in magnetic recording
US5309295A (en) * 1992-10-08 1994-05-03 International Business Machines Corporation Method and apparatus for biasing a magneto-resistive head

Also Published As

Publication number Publication date
JPS6215848Y2 (forum.php) 1987-04-22
JPS6121009U (ja) 1986-02-06
JPS52131708A (en) 1977-11-04

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